ESR News May 2015
Dual Energy CT: Technology and Dose Considerations
John Damilakis, Professor of Medical Physics
Dual energy computed tomography (CT) was first developed in the 1980s. Using conventional CT scanners, operators had to carry out two scans one after the other. This technique was not very successful due to high radiation dose, image registration and other problems. New CT technology allows simultaneous image acquisition at two energies. Materials can be distinguished based on their different energy attenuation profiles. Dual energy CT can differentiate structures with similar density but different composition.
Dual energy CT acquisition is possible by using either single source CT or dual source CT. In single source CT units, a generator switches X-ray tube potential from 80kVp to 140kVp corresponding to photon energies from about 40keV to 140keV. For each exposure, the exposure time is only 0.5msec, allowing simultaneous acquisition of low-kVp and high-kVp images. Dual source CT scanners are composed of two tubes and two detector arrays. The two tubes are positioned at 90 degrees from each other. For dual energy CT the potential applied across the two tubes is 80kVp to 140kVp. The tube load (mAs) is adjusted accordingly to 50mAs for the high-kVp tube and 200mAs for the low-kVp tube. Other approaches have been introduced through energy-sensitive detectors and photon counting detectors.
It is true that dual-energy scanning may be advantageous for several clinical cases. Dual energy CT is used for tissue detection and characterisation of renal stones, lung perfusion and ventilation, myocardial perfusion, and also in oncology. However, patient dose and associated risks are always a concern. Initial studies have shown that dual energy CT delivers higher doses to patients than single energy CT (1). Thomas and colleagues have shown that dual energy CT is associated with patient doses similar to those received during single energy CT (2). Recent studies report similar dose levels for dual energy CT as for single energy CT, or even lower. Kerl et al compared doses and image quality at coronary CT angiography between dual energy CT, dual source CT in single energy mode, and 16-slice CT. The authors found that dual energy CT delivers less dose than dual source CT or 16-slice CT without loss in image quality. Other studies have shown no significant difference in patient doses between dual energy CT and single energy CT. Henzler et al have recently reviewed the literature regarding radiation doses associated with dual energy CT applications (4). It should be noted that limited information is available in the literature. There are no sufficient data regarding patient doses from all dual energy CT techniques and further studies are needed to fully investigate this topic.
All CT examinations should be optimised to achieve diagnostic image quality with the lowest radiation dose possible. Dose optimisation of dual energy examinations is an area of great interest for both medical physicists and radiologists. The replacement of pre-contrast imaging by virtual non-contrast-enhanced imaging provides a great opportunity of radiation dose reduction. Moreover, several techniques and tools have been developed for CT dose optimisation and these methods are also applicable for dual energy CT studies. Thus, application of new iterative reconstruction algorithms, use of automatic exposure control and other dose saving tools may help to reduce patient doses considerably.
References
- Ho LM et al. Dual energy versus single energy MDCT: measurement of radiation dose using adult abdominal imaging protocols. Acad Radiol 2009;16(11):1400-1407
- Thomas C et al. Dual-enhanced CT for the characterization of urinary calculi: in vitro and in vivo evaluation of a low-dose scanning protocol. Eur Radiol 2009;19:1553-1559.
- Kerl JM et al. Dose levels at coronary CT angiography – a comparison of dual energy-, dual source- and 16-slice CT. Eur Radiol 2011;21:530-537
- Henzler T et al. Dual-energy CT: Radiation dose aspects. AJR 2012 ;199 :S16-S25